1. Field of Invention
The present invention relates to wireless information communications systems. More particularly, the present invention relates to an adaptive antenna array of a base station formed as a plurality of spaced apart clusters of antenna elements lying generally within a common horizontal plane.
2. Related Art
Wireless data and voice communications services are proliferating throughout the world. One popular service is the so-called "cellular" telephone service. In cellular telephone service, service areas are divided up into "cells", where each cell covers a specific geographical area and services mobile units located in, or passing through, the service area. Typically, radio frequencies are used in the ultra high frequency spectrum, and more typically in the 800 MHz or higher frequency range. The nature of radio wave propagation at these relatively short wavelengths limits the maximum effective distance between the mobile and the base, frequently to several miles. This propagation limit enables reuse of the same frequencies or bands within non-adjacent cells of the cellular network. Since the service range of each base station is limited to a radius of e.g. several miles, it is necessary to provide a number of base stations within a service area in order to provide effective wireless service throughout the area.
One known way to increase the number of mobile stations that may be served within a cell is to divide the cell into sectors, such as three sectors, spaced apart by 120.degree. about the compass rose. In such an arrangement, each sector is provided with its own 120.degree.-wide transmit beam from the base station.
Further increase in the number of mobile stations that may be simultaneously served within a cell or sector is to employ base station antenna arrays having plural elements. Embedding adaptive antenna array technology into the existing cellular telephone infrastructure potentially provides very significant capacity increases. This technology offers the ability to eliminate same cell interference for mobile stations being served simultaneously. It offers the prospect of a reduction of inter-cell interference. It also increases the signal-to-noise ratio of a particular mobile station being served and therefore enables an increase in user data rate. These benefits and advantages result in either higher data throughputs, or the ability to service more mobile stations simultaneously, within a given cell or service infrastructure. With spatially separated elements, beamforming becomes practical for both transmit and receive modes. Focusing radiant energy in the direction of a mobile station reduces the amount of overall power needed to be generated by the base station in order to maintain a given service quality. Antenna array technology can be used to focus power coming from the mobile station to the base station via a reverse link or an uplink, as well as from the base station to the mobile station via a forward link or downlink.
Usually, during transmit mode, a wide transmit beam is desired so that the transmit beam, and its associated pilot, reaches all of the mobiles within the service area or sector, since the base station does not initially know where any particular mobile would be within that area. In transmit mode, relatively wide transmit beams may be formed by using phased antenna elements of an array wherein the elements are spaced relatively closely apart, with spacing between adjacent elements being on the order of between one half and one wavelength at the transmit frequencies. At the cellular frequency bands in the range of 800 MHz, one wavelength equals 0.375 meters, or 14.775 inches, with one half wavelength being half of these linear values. After a particular mobile station is located within the service area of the base station, narrower transmit beams may be employed to divide and concentrate limited base station power among all of the mobile stations being served simultaneously.
In base station receive mode, very narrow beams are highly desirable in order to provide multiple beam diversities and concentrate the signal energy from a particular one of the mobiles operating within a particular one of the available service channels and to exclude or reduce signal energies from other mobiles within the same service area using other ones of the available service channels. Beamforming narrow beams in receive mode requires that the phased receive antenna elements be placed relatively farther apart than the transmit elements. Phased adjacent receive elements are most preferably placed apart by approximately three wavelengths. At 800 MHz, three wavelengths equals 1.125 meters or 44.325 inches. From these desirable spacings, it becomes immediately apparent that base station receive mode antenna arrays may become relatively quite large and visually noticeable at the base station locations within the neighborhoods of the various cellular communications service areas. Since the highest service requirements occur in the most highly populated areas, large base station antenna arrays become the subject of observation and complaint by a relatively large part of the population as a whole. One popular misconception held by some members of the public at large is that the larger the antenna array, the greater will be the exposure level to electromagnetic radiation at the vicinity of the array. Also, members of the public may object to what is perceived to be a negative visual impact or blight upon the environment of a particular neighborhood presented by large antenna arrays providing wireless communications services.
For example, FIG. 1 shows a conventional three-sector cellular antenna array 10 mounted at desired elevation above ambient terrain upon a triangular support tower 12. A triangular support tower is frequently employed in wireless communications because it provides considerable strength with minimal material and takes advantage of the inherent strength of three-leg, triangle geometry in the horizontal plane and triangle bracing in the vertical planes of each tower face. The antenna array 10 is designed to serve three service sectors 14, 16 and 18. For sector 14, a transmit-receive element 20 is located at one corner of the tower 12, and a receive-only element 22 is located at another corner of the tower 12 at a spacing selected to enable effective diversity reception. The antenna elements 20 and 22 are enclosed and protected from the weather by radomes 24, typically formed of radio-wave-transparent material such as molded fiberglass or plastic.
The transmit-receive element 20 is adapted to broadcast a service beam throughout the sector 14, and the element 20 may also be simultaneously used to receive at a different frequency or band with the inclusion of conventionally available duplexer filter technology, or may be used in a time division multiplex arrangement, with one time increment operating in transmit mode and a next time increment operating in receive mode. The receive mode element 22 provides spatial diversity reception for signals arising within the service sector 14. Similarly, the service sector 16 includes transmit-receive element 26 and receive-only element 28, and the service sector 18 includes transmit-receive element 30 and receive-only element 32. While the arrangement of antenna array 10 in FIG. 1 enables some beamforming, very narrow receive-mode beams with additional array gains of about 5 dB with respect to a single antenna element are not achievable with only two spatially diverse receive antenna elements.
Narrow beamforming creating very narrow beams with high antenna array gains at the base station for both receive and transmit modes typically requires more antenna elements. FIG. 2A presents a more recently proposed antenna array for wireless cellular communications service which employs a relatively large multi-element receive antenna array 50 and a relatively small multi-element transmit antenna array 52. While FIG. 2A shows the transmit array 52 to the side of the receive array 50, the transmit array 52 may also be mounted concentrically with the receive array 50 on a tower, provided that a different elevation is used to prevent the transmit array 52 from being blocked by the receive array 50.
The receive array 50 includes e.g. 16 separate receive elements 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, and 84 disposed along a circular locus formed by a support ring 85. The spacing between adjacent elements of the receive array 50 is preferably on the order of three wavelengths (3.lambda.). Each element 54-84 is provided with its own radome 86. As shown in the rectangular coordinate graph of FIG. 2B, the receive array 50 is capable of forming a relatively very narrow receive beam 88 in a particular direction within the service area relative to the array 50 with nearest adjacent side lobes 89 separated in phase from the beam 88 by approximately 22.degree.. The receive array antenna beam pattern shown directed to 180.degree. shown in FIG. 2B is a typical beam pattern that can be formed using the receive array 50. The rectangular coordinate graph of FIG. 2C shows a receive array beam pattern directed to 90.degree. and represents a typical beam pattern that can be formed using the receive array 50. In the graph of FIG. 2C, the nearest adjacent side lobes 89 are shown separated in phase from the main lobe 88 by approximately 22.degree..
The transmit array 52 includes e.g. 16 separate transmit elements 90, 92, 94, 96, 98, 100, 102, 104, 106, 108, 110, 112, 114, 116, 118 and 120, also disposed about a circular locus formed by a support ring 121. The spacing between adjacent elements of the transmit array is preferably on the order of one-half wavelength (1/2.lambda.) to one wavelength (1.lambda.). Because of the relatively close spacing, all of the transmit elements 90-120 may be enclosed within a common radome 122. As previously noted, the transmit array 52 may be located to the side of the receive array 50, or preferably above or below the receive array 50 in a concentric arrangement shown in dashed outline relative to the receive array 50 in FIG. 2A. The transmit antenna array 52 is arranged and operated to provide simultaneous transmit (downlink) signals for e.g. three sectors 124, 126 and 128.
FIG. 2D depicts a typical, relatively narrow transmit beam pattern having a main lobe 130 focused at a direction of 180.degree. with a maximum antenna gain (a main lobe 3 dB beamwidth at 17.degree., and side lobes 131 separated by 100.degree.) formed using the transmit array 52 of FIG. 2A. FIG. 2E depicts the relatively narrow transmit beam pattern formed by the transmit array 52 at a direction of 90.degree.. FIG. 2F shows a typical relatively wide transmit beam having a single lobe 132 directed at 180.degree. which may be formed by the transmit array 52. The beam of FIG. 2F has a 3 dB beamwidth of 120.degree., and is typically used for transmitting common pilot and broadcast channel information for the particular sector being serviced. With the transmit array 52 shown in FIG. 2A, each sector can be provided with a relatively narrow transmit beam 130 (FIG. 2E) and a relatively wide transmit beam 132 (FIG. 2F). The receive array 50 provides receive-mode (uplink) beamforming for all three sectors 124, 126 and 128, in the present example.
Spatial diversity multiple access methods employing adaptive antenna arrays are described in U.S. Pat. Nos. 5,471,647 and 5,634,199 to Gerlach et al., and methods and structures for providing rapid beamforming for both uplink and downlink channels using adaptive antenna arrays are described in commonly assigned U.S. patent application Ser. No. 08/929,638 to Scherzer, entitled "Practical Space-Time Radio Method for CDMA Communication Capacity Enhancement", all of which are incorporated herein by reference in their entirety.
While a number of benefits including increased service capacity can be realized by using adaptive antenna arrays, such arrays have heretofore been objected to by land use planning regulators because of concerns relating to perceived electromagnetic radiation hazards and concerns relating to objectionable or negative visual impact. Thus, an unsolved need has remained for a multi-element antenna array that provides such benefits while presenting a reduced visual impact at the base station location.